The present invention relates to an electronic converter for communication of analog information by wire and more particularly to an isolated two-wire transmitter.
At the present time a process plant, such as an oil refinery, includes a large number of separate devices or sensing points which are monitored from one or more central locations. For example, a central control room may monitor hundreds of sensing points each of which varies in some way. Each of the monitored devices or sensing points may have one or more transducers, such as temperature sensitive thermocouples, attached to them to convert the monitored effect, such as a temperature change, into an analog electrical signal. Those electrical signals often have too low a signal-to-noise ratio to be sent, over wires, directly to the central location. Consequently, the sensor (transducer) is connected by wires to a "two-wire transmitter" which is preferably located close to the transducer.
The two-wire transmitter receives the analog signals from the sensor, amplifies and isolates those signals to within a predetermined range of current, and sends the isolated current over wires to the receiving instrument in the control location. For example, the receiving instrument may be a controller, a warning annunciator, a recorder, or a meter, etc. The two-wire transmitter is powered by power from a direct current source connected to the same wires over which the transmitter sends its information to the central location.
There are a number of such two-wire transmitters (converters) presently available. In some cases they do not provide the requisite degree of isolation, for example, 100 volts isolation or less, and accuracy in that their output current is not accurate and truly linear of the input signal. In some isolated two-wire converters, although their initial accuracy may be satisfactory, they may become inaccurate due to environmental temperature variations, aging or other causes. In addition, in some cases there are limitations of space so that the relatively large size of some transmitters may present an installation problem. The cost of such transmitters may also be important since a single large process plant may utilize hundreds of such transmitters.
One type of isolated transmitter is proposed in U.S. Pat. No. 3,757,195 to Sklaroof. A different isolated transmitter is proposed in U.S. Pat. No. 3,483,476 to T. Kobayashi et al, in which the low-level input signal is chopped, which involves a relatively costly circuit. In Kobayashi a.c. signal is amplified and applied directly to an isolator transformer as a voltage signal using a two-tap primary and without current routing.